Known through DE 10 2006 033 087 A1 is a vehicle which has a drive train where a first clutch device is positioned between a combustion engine and a motor driven, a generator driven, electrical machine, and, in an 8-gear automatic transmission with four, partially coupled planetary gear sets, a second clutch device is positioned. During a start procedure via the electrical machine, the first clutch will be engaged and the second clutch, integrated into the transmission, will be activated to perform a temporary slippage operation. Thus, through this integrated starting clutch, the flow of force branches into several gears. A comparable parallel hybrid drive train with a slippage controlled, integrated starting clutch is also shown in DE 10 2006 005 438 A1.
The DE 198 18 108 A1 presents a different hybrid drive train, in which a combustion engine, an electrical machine, and a vehicle transmission are configured in a controllable drive relationship via a planetary transmission. On the input side, the combustion engine is connected with a planetary gear via a first shaft, and the electrical machine is connected via a second shaft with a sun gear of the planetary transmission. On the output side, a planetary gear carrier is connected with the vehicle transmission via a third shaft. A controllable clutch is positioned between the combustion engine and the electrical machine and which optionally connects the input shafts of the combustion engine and the electrical machine. To avoid a clutch kick during the start of the vehicle, the electric motor is operated initially, with a running combustion engine, with a disengaged clutch as a generator. Through its brake torque, the combustion engine is kept at a predetermined idle rotation speed via the planetary transmission, so that the rotational speeds of the three drive shafts converge and that, at equal rotation speeds, the clutch can be comfortably engaged, meaning absolutely kick free.
The permanent functional readiness of the shifting device, functioning as a starting clutch, is very important in such hybrid systems, especially in systems were the starting clutch is integrated into the vehicle transmission as a multifunctional part. However, a breakdown of such a controllable shifting device cannot be completely excluded.
If a vehicle is slowing down when driving at speed, with an engaged, integrated starting clutch, such clutch is expected to regularly disengage at a certain engine rotation speed, above or equal to a set idle rotation speed. If the clutch, despite a respective activation, does not disengage, for instance because of a hydraulic valve which is blocked, due to contamination in the hydraulic oil, such malfunction of the clutch can cause the engine rotation speed, during the slowdown of the vehicle, to end up lower than the set idle rotation speed. Hence, to avoid stalling of the engine, a usual idle control would request higher engine torque which will ultimately lead to acceleration of the vehicle. A driver, activating the brake pedal, experiences hereby an initial slowing down of the vehicle, followed by a surprising acceleration.
Such a behavior of a vehicle, which is irritating to the driver, can lead to a reduced drive comfort and can even result in a disabled vehicle.